In the last two posts I have tried to show that there is a benefit to running an occasional calibration test on equipment to ensure that it is giving the best performance. This does not mean that the nozzle needs to be tested every day, although some of the cheaper pressure washer nozzles, for example, will wear out in less than an hour. An operator will learn over time about how long a nozzle will last and can, after a while, tell when it is starting to lose performance. But in working on a number of different jobs in succession, that sense of the performance may be missed, and it can be handy to have a standard target that a jet can be pointed at and that it should be able to cut in a known time.

One simple target is plywood, and, to continue the saga of nozzle comparisons through a slightly different approach, Mike Woodward used plywood sheets to compare different nozzles in one of the earliest comparisons of performance. We since duplicated his test equipment and ran tests with a more modern selection of nozzles but the basic results and conclusions remain the same.

In its simplest form, the idea is to build a holding frame that will hold small squares of plywood at fixed distances from the nozzle. In the frame shown below, the plywood pieces are set at one-foot distances apart with the nozzle held at a fixed point at the end of the test frame. Tests showed that it takes around 2,700 psi to cut through the plywood.

Figure 1. A simple frame to hold plywood samples

The initial tests that Dr. Woodward ran were run on nozzles that were run at 10,000 psi with a nominal flow rate of 10 gpm. The nozzles that were used cost in the range from $10.00 to $250 a piece. (And these costs were reported in 1985 at the 3rd American Waterjet Conference). Tests such as this are simple to run. Plywood pieces are set into the frame, the nozzle is placed at the end of the frame and the jet run for ten seconds. Over that time, the jet will cut through any of the pieces of plywood that it reaches with enough power to cut through, and generally, the jet will punch a hole through several pieces.

Figure 2. The different designs of nozzle that Mike Woodward tested in 1985

The profiles show that there was only one of the common nozzles at the time that fitted smoothly onto the end of the feed pipe. In the other cases, there is a small gap between the nozzle piece and the feed tube so that turbulence would be generated just as water entered the acceleration section of the nozzle.

The hole size in each plate was then measured and that width plotted as a function of the distance from the nozzle so that a profile of the jet cutting path could be drawn.

Figure 3. Profiles cut into the different pieces of wood showing the cutting power of the different jets as a function of distance and the actual amount of water flow as measured

As an additional part of the testing, a rough measure was kept of the effective nozzle life. Some other performance parameters for the different nozzles can be put into a table.

Figure 4: Performance of the different nozzles

Clearly, just going out and buying the most expensive nozzle on the block is not necessarily the best idea. But it also depends on the use to which the nozzle is going to be applied. There are two different applications: that of cleaning a surface and that of cutting into it. The broader path achieved by nozzle 1, for example, which also removed the largest volume of wood per horsepower, makes it a good selection for cleaning and for reaching further from the nozzle as would be needed if one were cleaning the pipes of a heat exchanger bundle.

On the other hand, the more coherent flow through nozzle 2, which gave a narrower cut, might be a more effective tool in a cutting operation. In other cleaning operations, where the nozzle is being operated very close to the surface, then nozzle 3, which has a wider path, might be a better choice, though that is lost if the target surface is further away. And though there was not a great deal of difference in performance between nozzles 1 and 5, there is a considerable difference in price.

A smaller, lighter nozzle may be a beneficial trade-off if the nozzle body is fitting on the end of a lance that will be operated manually for several hours at a time.

There is an alternate way of using plywood as a target that I have also used in teaching class. The student is using a manually operated high-pressure cleaning gun at 10,000 psi and is to swing the gun horizontally so that the jet cuts into a piece of plywood that is set almost parallel with the jet path, but with the stream hitting the wood from the side initially further from the operator. But as the swing completes the jet cuts up where the nozzle almost touches it and then sweeps on past.

The result is that, over the distance, the jet can cut into the wood and a groove is carved into it.

Figure 5. Horizontal cuts into plywood. There were about half-a-dozen students who had swiped the nozzle so that it just cleared the left edge of this 4-ft wide piece of plywood, and you may note that the cuts extend roughly ¾ of the way along the surface

Once the students had seen this cut, I would ask them how far away they thought, based on that measurement, the jet would cut into a person. Typically they said about three feet, and then, as a precaution, I suggested they add a foot or so more.

Then I took them over to a metal frame where we had hung a piece of pork. We carefully measured off the “safe” distance from the end of the nozzle to the pork.

“Now assume that is you”, I would say, “swing the jet as fast as you can, so that it barely has time to hit “your arm”, and we’ll just check that distance is correct.”

Figure 6. Piece of pork that has been traversed by a 10,000 psi jet several times, with a typical stand-off distance from the nozzle of more than four feet.

Invariably we got the result shown in Figure 6. The jet would cut into the meat to a typical depth of around two inches and groove the underlying bone. It was a salutary way of getting their attention about the safe use of waterjet technology, and I noticed that the staff also got a bit more cautious after we ran this class every year.

Last week’s post discussed a simple test which helps to show not only how to compare the effect of different operating conditions (varying abrasive type, nozzle design, AFR etc) as a way of finding a possibly better and cheaper cut. It is also often handy to know when a nozzle is starting to wear out, so that different cutting operations might be scheduled to allow the nozzle to continue to work, without threatening the quality of a critical product.

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use

While we have found that nozzles from a given manufacturer roughly agree in cutting performance and times before they wear out, the pattern of wear and performance change differs from one nozzle design to another. Also there is some variation in performance between nozzles even of the same design and under the same conditions.

There are also times when cuts are made without abrasive or when the cutting/cleaning jet is hand-held – what to do in those cases? Mainly we have used foam as the cutting target, set up so that the jet won’t cut all the way down through the foam all the way along the cut, so that, as with the steel, some idea of not only cutting depth but also cut quality can be seen.

Figure 2. Cuts through thick stiff packing foam. Note the rough edge at the bottom of the extracted pieces, but the good initial quality of cut that was achievable for some 14-inches.

There is a caution in cutting foam in that some of the softer varieties are going to fold into the cut and give a slightly inaccurate measure of true performance; although for a quick comparison to see how a nozzle is lasting that is not a real issue. When cutting thicker material and also when going for higher quality cuts that is, however, something that should be borne in mind.

The white expanded foam that is used as a packing material is also very easy to cut, even with the pressures that can be found with a pressure washer type of system. Thus, if you are going to clean a deck or other surface, it helps to check by swiping the jet across such a piece of material to be sure that you have a good nozzle on the end of your lance before you start.

This may seem fairly logical; after all you just went to the hardware store and bought a new packet of nozzles. Well, as with the other nozzles we have looked at, quality is only assured after testing. In this particular case, we ran as many different varieties of fan nozzles as we could to see how they would perform when cutting across a piece of packing foam. It is not hard to cut packing foam with a high pressure jet. And since domestic cleaning is usually carried out at either 1,000 psi or 2,000 psi, we ran tests at both levels.

Figure 3. Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches. The number identifies the nozzle. Note that at 3 inches, number 18 could barely remove the top of the foam.

A fan jet is defined by the amount of water that it will allow to pass at a set pressure and by the angle of the cone with which the jet spreads out from the orifice. In passing, we found that the cone angle that the jet actually spread at was a little larger than that designated on the package.

The worst nozzle design that we found had difficulty in cutting into the foam even at a very close range:

On the other hand, the best nozzle was still able to cut the material with the nozzle held some nine inches from the foam.

Figure 4. Cutting result with the good nozzle held at nine inches above the foam target. At this distance the jet is removing as much material as the poor jet did at a 3-inch standoff.

A very typical result would have the jet fail to cut into the foam much beyond four inches from the nozzle. (I’ll use some photographs in a couple of weeks to explain in more detail why that is). And as a short editorial comment to those of you who clean around your house with a domestic unit, how many of you hold the nozzle that close to the surface? (Or at the car wash?) If you don’t you are losing most of the power that you are paying for and you are in the company of most of the students that I ran this demonstration with in my classes).

However there is one other feature to the photographs of the cuts that I would point out. Fan jets distribute the water over a diverging fan shape. But the results of the design fell into two different types, one where most of the water still concentrated in the middle of the jet, (as in Figure 4) and those where it was focused more on the side.

Figure 5. Cutting pattern with the jet streams more at the side of the flow (arrow points). Note that the two pressure cuts are on the other sides of the sample here.

The benefit of using foam is that it allows this picture of the jet structure to be easily seen, with very little time taken to swipe the nozzle over a test piece of material at the start of work, to make sure that the jet is still working correctly.

This is both an advantage and a disadvantage. Because the foam is relatively easy for a jet to cut even at a lower pressure, this means that the cut can become more ragged with depth where deep cutting is required.

One of the programs that we ran, some years ago, looked at how deeply you could cut into the stiff packing foam that is used in some industrial plants, where the item being packed needs to be held firmly yet will be released easily when needed. This requires that the foam be cut to a very tight tolerance, and at the time, pieces were still being cut by hand and then glued together. (Figure 2 above)

We found that we could cut up to about a foot of material before the small cut particles became sufficiently caught up in the cutting jet that the edge quality of the cut fell below specification. But in order to get to that depth we did have to add a small amount of a polymer to the cutting water. This helped to hold the jet more coherent over a greater distance and also reduced the amount of particulate that got caught up in the jet allowing the greater cutting depth.

Foam works as a simple sample to give some sense of the jet shape where the pressures are lower. When they are higher, a stiffer material is needed though it should still be cut by water without the need for abrasive. Plywood is a useful target in this case, and I will write about those tests next time.

This post is being written in Missouri, and while the old saying about “I’m from Missouri, you’re going to have to show me,” has a different origin than most folk recognize*, it is a saying that has served well over the years. We did some work once for the Navy, who were concerned that shooting high-pressure waterjets at pieces of explosive might set them off as we worked to remove the explosive from the casing. We ran tests under a wide range of conditions and said, in effect, “see it didn’t go off – it’s bound to be safe!” “No,” they replied, “we need to know what pressure causes it go off at, and then we can calculate the safety factor.” And so we built different devices that fired waterjets at pressure of up to 10 million psi, and at that pressure (and usually a fair bit below it) all the different explosives reacted. And it turned out that one of the pressures that had been tested earlier was not that far below the sensitivity pressure of one of the explosives.

That is, perhaps a little clumsily, a lead in to explain why simple answers such as “yes I can clean this,” or “yes I can cut that” often are not the best answers. One can throw a piece of steel, for example, on a cutting table and cut out a desired shape at a variety of pressures, abrasive feed rates (AFR) and cutting speeds. If the first attempt worked, then this might well be the set of cutting conditions that become part of the lore of the shop. After a while it becomes “but we’ve always done it that way,” and the fact that it could be done a lot faster with a cleaner cut, less abrasive use and at a lower cost is something that rarely gets revisited.

So how does one go about a simple set of tests to find those answers? For many years, we worked on cutting steel. Our tests were therefore designed around cutting steel samples because that gave us the most relevant information, but if your business mainly cuts aluminum, or titanium or some other material, then the test design can be modified for that reason.

The test that we use is called a “triangle” test because that is what we use. And because we did a lot of them, we bought several strips of 0.25-inch thick, 4-inch wide ASTM A108 steel so that we would have a consistent target. (Both quarter and three-eighths thick pieces have been used, depending on what was available). The dimensions aren’t that important, though the basic shape that we then cut the strips into has some advantage as I’ll explain. (It later turned out that we could have used samples only 3-inches wide, but customs die hard, and with higher pressures the original size continues to work).

Figure 1. Basic Triangle Shape for waterjet test cutting

The choice to make the sample 6-inches long is also somewhat arbitrary. We preferred to make a cutting run of about 3 minutes so that the system was relatively stable, and we had a good distance over which to make measurements, but if you have some scrap pieces that can give several triangular samples of roughly the same shape, then use those.

The sample is then placed in a holder, clamped to a strut in the cutting table and set so that the 6-inch length is uppermost and the triangle is pointing downwards.

Figure 2. The holder for the sample triangle

The nozzle is placed so that it will cut from the sharp end of the triangle along the center of the 0.25-inch thickness towards the 4-inch end of the piece. The piece is set with the top of the sample at the level of the water in the cutting table. The piece is then cut – at the pressure, AFR and at a speed of 1.25 inches per minute with the cut stopped before it reaches the far end of the piece, though the test should run for at least a minute after the jet has stopped cutting all the way through the sample.

The piece is then removed from the cutting table and, for a simple comparison, the point at which the jet stopped cutting all the way through the triangle is noted.

Figure 3. Showing the point at which the jet stopped cutting through various samples as a function of the age of the nozzle – all other cutting conditions were the same (a softer nozzle material was being tested which is why the lifetime was so short). The view of the samples is from the underside (A in Fig 1.)

An abrasive jet cuts into material in a couple of different ways – the initial smooth section where the primary contact occurs between the jet and the piece and the rougher lower section where the particles have hit and bounced once on the target, and now widen and roughen the cut. Since some work requires the quality of the first depth, we take the steel samples, and mill one side of the sample, along the lower edge of the cut until the mill reaches the depth of the cut, and then we cut off that flap of material so that the cut can be exposed. Note that the depth is measured to the top of the section where the depth varies.

Figure 4. Typical example of a steel triangle that has been cut and then sectioned to show the quality of the cut

I mentioned in an earlier article that we had compared different designs from competing manufacturers. Under exactly the same pressure, water flow and abrasive feed rates, the difference between the cutting results differed more greatly than had been expected.

Figure 5. Sectioned views of six samples cut by different nozzle designs, but at the same pressure, water flow, AFR and cutting speed

There was sufficient difference that we went and bought second and third copies of different nozzles and tested them to make sure that the results were valid, and they were confirmed with those additional tests. Over the years as other manufacturers produced new designs, these were tested and added into the table – this was the result after the initial number had doubled. (The blue are results from the first nozzle series tests shown above).

Figure 6. Comparative depths of cut using the same pressure and AFR but twelve different commercially available nozzle designs

There were a number of reasons for the different results, and I will explain some of those reasons as this series continues, but I will close with a simple example from one of the early comparisons that we made. We ran what is known as a factorial test. In other words the pressure was set at one of three levels and the AFR was set at one of three levels. If each test ran at one of the combination of pressures and AFR values and each combination was run once then the nine results can be shown in a table.

Figure 7. Depths of cut resulting from cutting at jet pressures of 30,000 to 50,000 psi and AFR of 0.6, 1.0 and 1.5 lb/min

The results show that there is no benefit from increasing the AFR above 1 lb/minute (and later testing showed that the best AFR for that particular combination of abrasive type, and water orifice and nozzle diameters was 0.8 lb/minute).

Now most of my cutting audience will already know that value and may well be using it but remember that these tests were carried out over fifteen years ago, and at that time, the ability to save 20% or more of the abrasive cost with no loss in cutting ability was a significant result. Bear also in mind that it only took 9 tests (cutting time of around 30 minutes) to find that out.

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* The reason that the “I’m from Missouri, you’ll have to show me,” story got started was that a number of miners migrated to Colorado from Missouri. When they reached the Rockies they found that, though the ways of mining were the same, the words that were used were different. (Each mining district has its own slang). Thus they asked to be shown what the Colorado miners meant, before they could understand what the words related to.

In the next few posts I will be writing about some of the tests that you can run to see how a nozzle is performing. But before getting into the details of the different tests, you should recognize that this is where a little homework will be required if you are to get the most benefit from the topic.

The world that encompasses waterjet use has grown beyond the simple categories by which we used to define it. New techniques make it possible to cut materials that used to be more difficult and expensive to produce, and as practical operational pressures have increased, so the scale, precision and economics of new opportunities have developed.

It is this range of applications that makes it impractical for me to give specific advice for every situation. So instead, by explaining how to make comparisons and what some benchmarks might be, I try to allow you to better understand your system, its capabilities and both the initial performance of nozzles. Hopefully, you’ll then be able to evaluate and decide when they may best be replaced.

One lesson I learned early was that nozzles from different companies behaved in different ways and that drawing conclusions on optimal performance, for example the selection for which pressure level and nozzle size was best, using one design would not necessarily hold with a competing design. Further, there were nozzles that began their life on our system doing very well relative to others, but which quickly declined in performance. Thus, as part of an evaluation of different designs, we would test the nozzle cutting performance against a standard requirement at fixed time intervals so that we would know when it was wearing out and should be replaced.

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use.

Both the shape of the curve and the effective lifetimes of different competing nozzle designs varied quite significantly. And obviously, since most folk don’t spend a lot of their time cutting through more than an inch of steel, the operational lifetimes of nozzles will vary with the requirements for the particular job. Nevertheless, the relative ages at which nozzles can no longer reach that target can differ significantly.

Figure 2. Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

As mentioned, the tests were carried out using nozzles from several manufacturers, and at the beginning of the test, the longest lasting nozzle was not necessarily the one that produced the fastest cut, but consistently over the interval and for about twice as long as the competition, it was able to achieve the goal.

Figure 3. Depths of cut in steel after (top) 1,000 minutes of nozzle use, and (bottom) after 1,500 minutes of nozzle use.

In the particular case in which we made the comparison, the major interest was in achieving a clean separation of the parts, and the edge quality was not as significant a factor. In many uses of this tool that edge quality will be important and would have given a different set of numbers (as Figure 3 would indicate) than the ones that were found for our application. As a result, the judgment that the nozzle is worn out will change to a different time, and the relative ranking of the different nozzle designs may also change.

The only way in which anyone can make a rational decision on which is the best nozzle for an application and how long it will be effective is by testing the nozzle against the stated requirement. When we began the test, we anticipated that the difference between nozzles from different manufacturers when fed with water at the same flow rate and with the same quantity and quality of abrasive would not differ that much. As Figure 2 shows, we were wrong in that idea.

There are a number of different impacts that a change in nozzle design (i.e. in most cases buying a competing design over that initially used) can bring to a cutting operation. However, these impacts are also governed by the pressure at which the work is being carried out, the amount of abrasive that is used, the relative nozzle diameters (if using a conventional abrasive waterjet system) and the speed at which the cut is made. But an initial assessment of relative merit should be carried out with equivalent parameters for the different designs.

In general, however, we ran tests at a number of pressures and with varying abrasive feed rates to ensure that the comparative evaluations were fair and consistent. As a result, we found that there were a number of different factors that came into play which are not always recognized and which could bias the results that we observed.

In the posts that follow this, I will first cover some of the different tests that can be used and then go on to explain some of the results and why they sometimes make it difficult to accept a simple comparison of results when, for example, the abrasive is not the same in both cases. To give a simple example of this, consider a conventional abrasive waterjet nozzle that is operated at increasing pressure.

Increasing the pressure will improve the cutting speed and/or the cut quality, as a general rule. It will reduce the amount of abrasive that is needed but this is where the “yes, but’s . . . .” start to appear. As the pressure of the jet increases, so the amount of abrasive that is broken within the mixing chamber will also increase so that the average size of the particle coming out of the nozzle will become smaller. The amount of this size reduction is a function of the quality of the abrasive that is being used and a function of the initial size of that abrasive.

Within a certain size range, that reduction in the particle size does not significantly change the cutting performance, but if the mix contains too many small particles, particularly if the distance to the work piece is also significant, then the cutting performance can be reduced because of the particle break-up. Different nozzle designs produce different amounts of very fine material even from the same feed rate of the same abrasive into the nozzle. When the initial feed rate of the abrasive or a different abrasive is used, estimating which design and set of operating pressures is best becomes more difficult as an abstract estimation.

This is why, in the posts that follow, the comparisons are made are based on actual measurements and why I recommend that everyone test their system using more than one design/set of operating parameters so that they can be confident that the combination that they are using will provide the best combination for the job to be done.